An international team of astronomers from Chile, Europe, the United States, Canada and New Zealand has captured the most detailed spectroscopic glimpse yet of an interstellar comet traversing our Solar System. Using spectroscopic data from two instruments on ESO’s Very Large Telescope (VLT), they’ve detected atomic nickel and cyanogen gas emission from 3I/ATLAS, only the third confirmed interstellar object on record.
This image of the interstellar comet 3I/ATLAS was captured with Hubble’s Wide Field Camera 3 (WFC3) instrument on December 27, 2025. Image credit: NASA / ESA / CSA / Hubble.
3I/ATLAS was discovered by the NASA-funded ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope on July 1, 2025.
Also known as C/2025 N1 (ATLAS) and A11pl3Z, the interstellar visitor arrived from the direction of the constellation Sagittarius.
At the time, the comet was at a heliocentric distance of 4.51 astronomical units (AU), with an eccentricity of 6.13.
“Characterizing the volatile composition of interstellar objects passing through the Solar System provides a unique window onto the chemical and physical processes operating in distant stellar systems,” said Dr. Rohan Rahatgaonkar from the Pontificia Universidad Católica de Chile and colleagues.
“Interstellar objects preserve signatures of the chemical and physical processes operating in their natal protoplanetary disks, potentially modified by exposure to the interstellar medium.”
“When heated by solar radiation, cometary interstellar objects release solids and gas due to cometary activity.”
In July and August, the astronomers conducted a campaign of high-resolution spectroscopy as 3I/ATLAS reached distances between about 4.4 and 2.85 AU from the Sun.
To obtain the spectra of the comet, they used VLT’s X-Shooter and the Ultraviolet and Visual Echelle Spectrograph (UVES) instruments.
The observations show that the comet’s coma — the hazy cloud of gas and dust that envelops its nucleus — is dominated by dust, with a consistently red optical continuum suggesting organic-rich material.
This reddish tint is reminiscent of both solar system comets and some of the most primitive bodies in the Kuiper Belt, hinting at shared physical processes across planetary systems.
Continuum-subtracted UV/blue spectra of 3I/ATLAS showing Ni I emission over 11 VLT/X-Shooter (blue) and two VLT/UVES (cyan) visits. Image credit: Rahatgaonkar et al., doi: 10.3847/2041-8213/ae1cbc.
As 3I/ATLAS carried closer to the Sun, the researchers detected emission from cyanogen (CN) — a simple carbon-nitrogen molecule commonly seen in cometary atmospheres — and numerous lines of neutral nickel (Ni).
In contrast, iron (Fe) was not detected, suggesting that nickel is being efficiently released from dust grains in the coma under the influence of solar radiation.
The scientists found that production rates of these species increased steeply as the comet approached the Sun, with the evolution of CN and Ni emissions following strong power-law relationships with heliocentric distance.
These trends suggest that the processes liberating these atoms may involve low-energy mechanisms such as photon-stimulated desorption or the breakdown of complex organics, rather than straightforward sublimation of ices — a nuance that sets this interstellar visitor apart from many solar system comets.
This spectral fingerprint provides more than just a snapshot of a fleeting visitor.
Interstellar comets like 3I/ATLAS are pristine samples of material forged around other stars. Because they have not been deeply processed by repeated trips around a Sun, they preserve clues about the composition of distant protoplanetary disks — the swirling disks of gas and dust that give rise to planets.
Previous interstellar objects, such as ‘Oumuamua in 2017 and 2I/Borisov in 2019, revealed surprising diversity — ‘Oumuamua behaved more like an inert rock, while 2I/Borisov was rich in carbon monoxide and complex ices.
This new study suggests that 3I/ATLAS adds yet another chapter to this growing cosmic story: a dust-rich body that reveals molecular signatures in ways that blur the line between familiar cometary behavior and new physics.
Continuum-subtracted spectra of 3I/ATLAS in the CN violet band region from a monitoring campaign spanning July 4-August 21, 2025. Image credit: Rahatgaonkar et al., doi: 10.3847/2041-8213/ae1cbc.
“If 3I/ATLAS continues to exhibit Ni without Fe through perihelion, it will constitute the first clear case where interstellar cometary metal emission is decoupled from classical refractory release,” the authors said.
“That outcome would argue for a distinct low-temperature organometallic (or nanophase) pathway for Ni in extrasolar comets and could open a new window onto how disk chemistry, metallicity, and irradiation history imprint on planetesimal microphysics.”
“While the parent star of 3I/ATLAS is likely to be a metal-poor relative to other progenitor stars of interstellar objects, it is unlikely to be even a factor of 2 less metal-rich than the Sun, meaning that there is no tension between the inferred age of 3I/ATLAS and the presence of an iron peak element like Ni.”
“Regardless of which scenario prevails, 3I/ATLAS offers a decisive time-critical experiment linking metal emission to volatile activation and grain physics in an interstellar object.”
“The measurements outlined above will turn nickel from a curiosity into a calibrated tracer of both parent chemistry and Galactic provenance, setting the standard for rapid-response spectroscopy of interstellar objects in the Rubin Observatory and ESO Extremely Large Telescope era.”
The findings were published December 10, 2025 in the Astrophysical Journal Letters.
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Rohan Rahatgaonkar et al. 2025. Very Large Telescope Observations of Interstellar Comet 3I/ATLAS. II. From Quiescence to Glow: Dramatic Rise of Ni i Emission and Incipient CN Outgassing at Large Heliocentric Distances. ApJL 995, L34; doi: 10.3847/2041-8213/ae1cbc
